Why can't I send sms messages on my ipad

SMS messages are sent via a cellular provider's text messaging plan and that is only available on phones. About iMessage and SMS/MMS - Apple Support

If you have an iPhone, you can however use Continuity to send an SMS message from the iPad through the iPhone. See: Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

Because an iPad cannot connect to any cellular telephone voice network, and SMS and MMS are sent over part of the telephone voice spectrum, not the data spectrum nor the internet.

SMS/MMS are very old technologies nowadays, and actually pre-date the internet. They use a small portion of unused bandwidth in the cellular telephone voice spectrum, and thus requires a cellular telephone. A data only device like an iPad cannot connect ot the necessary spectrum to use SMS/MMS natively.